The present invention relates to a machine having a moveable machine part and an air bearing supporting the moveable machine part. Moreover, the invention relates to an advantageous method for operating such a machine.
Many machines, in particular machine tools and coordinate measuring machines, have a displaceable working head. In the case of a coordinate measuring machine, on which the following will be based by way of example, the head is often arranged on the lower free end of a vertically arranged quill. The quill is displaceable, so that the head can be displaced relative to a measurement table. The measurement table is designed to support a measurement object. The quill is often arranged on a crossbar of a gantry, and it can be displaced along the crossbar in a first horizontal direction by means of a carriage. The gantry can be displaced together with the quill in a second horizontal direction, so that the measurement head can be displaced overall in three mutually perpendicular spatial directions. The quill, the carriage and the gantry form a frame structure. The maximum displacement paths of the measurement head along the three movement directions define a measurement volume, inside of which spatial coordinates on a measurement object can be determined.
In order to carry out a measurement, the measurement object is arranged in the measurement volume of the coordinate measuring machine. This is done in a first spatial position of the measurement object, relative to the measurement head. The measurement head is subsequently displaced to selected measurement points on the measurement object, in order to survey the measurement object. To this end, the coordinate measuring machine comprises actuators which move the gantry, the carriage and the quill in the respective spatial direction when actuated. Stepper motors or servomotors are typically used as actuators, since these allow very exact positioning of the measurement head. Since a very high accuracy is required when displacing the measurement head in coordinate measuring machines, the moving parts of the frame structure are supported without friction with respect to one another by means of air bearings. Air bearings, or aerostatic bearings, produce an air gap between the parts supported with respect to one another by blowing compressed air between the parts. In this way, an air cushion is formed between the parts supported with respect to one another, which prevents friction between these parts.
DE 34 41 426 A1 describes a coordinate measuring machine in gantry design, which comprises air bearings. During operation, compressed air from a compressed air source is to be blown into the air bearing through a tube and a restrictor bore. The compressed air flows through between the components supported with respect to one another and, owing to its pressure effect, creates a spacing in the form of a gap between the components. When this achieved, the gantry can be displaced so that a measurement head arranged on the gantry can be displaced with respect to a measurement object.
Typically, air bearings are constantly supplied with compressed air during operation of a machine, so that the head can be displaced in order to survey a measurement object or in order to process a workpiece. During times when the machine is static, the air bearings continue to be supplied with compressed air in order to prevent damage. This would happen if the air bearings are insufficiently supplied with compressed air and the head is displaced. In this case, the parts to be supported with respect to one another rest on one another, so that friction occurs between the parts. If the head is displaced in this situation, this leads to damage between the parts owing to the friction. It can also cause damage to the actuators, which may be overloaded because of the friction. In the event of long static times, the disadvantage arises that the measuring devices are contaminated by the compressed air. This happens in particular at abutting surfaces of multi-part measuring devices.
Switching off the machine completely during static times has the disadvantage that the machine has to be switched on again for renewed operation. This requires a series of initialization steps. First, the air bearings have to be supplied sufficiently with compressed air. Then, a present position of the head needs to be determined for control. Furthermore, the control must be adapted to present thermal conditions of the machine. Such initialization steps require time, which increases the static time of the machine and thereby reduces the availability of the machine.
German utility model registration G 80 31 939.7 proposes a height measurement and marking apparatus, which has a displaceable apparatus base. For displacement, the apparatus base is supported on an air cushion which is supplied by a compressed air feed. When the compressed air feed is interrupted, the apparatus base is lowered onto an aligning plate. In order to allow the apparatus base still to be moved smoothly, rollers are provided on the lower side of the apparatus base, which rest on the aligning plate after the step of lowering.
U.S. Pat. No. 3,943,972 discloses a pneumatic circuit for industrial applications. A compressed air feed is connected via switching valves to pneumatic cylinders. The switching valves comprise solenoid switches, which determine a switching position of the switching valves. By actuating a manual switch, the switching positions can be modified so that the cylinders are supplied with compressed air. Furthermore, a pneumatically controlled switch-off valve is arranged between a compressed air source and the cylinders, the switching position of said switch-off valve depends on a control pressure. This switch-off valve can provide a compressed air feed to the cylinders or interrupt it. In order to control the switching position of the switch-off valve, a delay component in the form of a compressed air chamber and a compressed air restrictor is provided. When the manual switch is actuated, the delay component is supplied with compressed air. If the manual switch is not actuated, air escapes from the delay component so that an air pressure inside the delay component decreases and therefore the control pressure at the switch-off valve is reduced, until the switching position of the switch-off valve changes. A timeout is therefore produced between the compressed air source and compressed air cylinders.
EP 0 546 694 also discloses a pneumatic circuit which switches off a compressed air feed to a cylinder after a certain time. This time delay is likewise based on the use of a delay component which is established in the form of a compressed air chamber and a restrictor.
During operation of such machines, safety devices are often used for protecting the user against hazards and/or the machine against damage. For safety reasons, actuators may not be automatically switched on after a complete switch-off. Usually, compressed air monitoring means are used in order to completely switch off the entire machine in the event of a pressure drop or lack of compressed air flow. Error monitoring means are furthermore used, which detect an excessive current in electrical actuators or position lags, i.e. errors between desired and actual positions. Response of each of these safety devices typically leads to the machine being switched off and countermeasures for restarting the machine have to be carried out by a user. The static time of the machine is therefore increased until the countermeasures are completed. For example, interrupting the compressed air would mean that the air monitoring switches off the machine when the air bearings are undersupplied with compressed air in order to prevent damage. Automatic restart of the machine is not possible then, so that a user has to restart the machine manually and carry out the initialization steps mentioned above.
During the initialization steps, a current position of the head may not coincide with an assumed position of a closed-loop position controller for the head. In the initialization, this may lead either to the closed-loop position controller having to be reinitialized or to the head being moved abruptly into the desired position specified by the closed-loop position controller. This can cause position lags, which lead to measurement errors or defective workpieces.